1. Field of the Invention
The present invention relates to a direct microwave modulation and demodulation device. The device is designed to be in the form of an integrated circuit and is particularly intended for data processing in the field of microwave link communications.
Since the circuits used for direct modulation and demodulation are virtually identical except for minor details, the present invention shall be explained with reference to a demodulator for the sake of clarity. Details that are specific to a modulator shall be given where necessary.
2. Description of the Prior Art
Classical circuit designs for digital microwave link communications receivers always include circuits operating at intermediate frequencies. These intermediate frequency circuits carry out the preamplification, filtering and amplification with automatic gain control (AGC) necessary to provide a coherent, constant amplitude signal to the demodulator.
The principle of direct modulation, which is known in the art, consists in transposing the intermediate frequency operations to a base band and regenerating the modulated carrier in the demodulation stage. Compared with classical approaches, direct modulation obviates the need for a number of circuits, including: 3 dB couplers, 90 degree phase shifters, a voltage-controlled oscillator, a low-noise AGC amplifier, and mixers.
In prior art direct demodulation, a local oscillator delivers a single signal that supplies two phase shifter stages. A first phase shifter accordingly produces an in-phase signal P (0 degrees) and a quadrature signal Q (90 degrees). In a second stage within the mixer, a second phase shifter converts the in-phase signal into two signals that are 0 and 180 degrees out of phase, while a third phase shifter converts the quadrature signal into two signals that are 90 and 270 degrees out of phase.
Also, prior art demodulators are generally in the form of hybrid circuits in which phase shifts are obtained by means of delay lines having various propagation modes; microstrip lines, slotted lines, coplanar lines.
If monolithic gallium arsenide integrated circuit technology is used, the length of the delay line becomes prohibitive, and it is therefore necessary to turn to other designs, e.g. employing localized elements.